240 AN INTRODUCTION TO ZOOLOGY 



by means of hard tendons. Muscular action is either voluntary 

 or involuntary; for example, the jaws and wings are moved by 

 voluntary, many internal organs by involuntary, muscles. The 

 strength of the muscles of the bee is much greater than that of 

 the muscles of man, compared with their weight. The explana- 

 tion of this is quite simple, since the weight of a muscle increases 

 as the cube of its diameter, while its strength increases only 

 as the square. A large animal can pull, therefore, comparatively 

 less than a smaller one. 



THE DIGESTIVE SYSTEM (Fig. 134). The digestive canal is 

 made up of the following structures named in order, beginning at 

 the anterior end: the mouth, oesophagus or gullet, honey sac or 

 honey stomach, true stomach, small intestine or ileum, and large 

 intestine or colon. It opens anteriorly by the mouth and pos- 

 teriorly by the anus. The (esophagus (ess.) is a narrow tube 

 which passes through the thorax; its posterior end is enlarged 

 into the honey sac (hs.) situated near the anterior end of the abdo- 

 men. At the posterior end of the honey sac is the stomach- 

 mouth (p); this structure extends slightly forward into the 

 honey sac. It has four triangular lips, which may be opened or 

 closed by two sets of muscles, longitudinal and circular. Near 

 the top of the lips are a number of bristles which project back- 

 ward. If the alimentary canal of a freshly killed bee is placed in 

 a f to per cent salt solution, the lips will open and close 

 rapidly for about half an hour. 



The true stomach (c.s.) is a cylindrical sac; its walls contain a 

 number of circular muscles, and a layer of longitudinal muscles. 

 The digestive juices secreted by its walls change the food into 

 chyme. Part of this chyme is absorbed, the rest is forced by 

 muscular contractions into the small intestine. Undigested food 

 is dissolved in the small intestine, or ileum (si.), and the digested 

 food or chyle is absorbed by its walls. At its posterior end the 

 small intestine gradually merges into an enlargement, the colon (I). 

 This part of the alimentary canal receives all the undigested 

 matter, and discharges it to the outside through the anus. The 



